1. Field of the Invention
The present invention relates to a Cu (copper) precipitation strengthened steel suited for use as a material for the construction of large industrial machines, ships, marine structures, line pipes, tanks, bridges and like welded structures, and to a method of producing the same.
2. Description of the Related Art
Recent years have seen continuous increases in the strength of welded structures, such as large industrial machines, ships, marine structures, line pipes, tanks and bridges, as requested from the viewpoint of economy and safety. To keep up with such trend, higher and higher levels of characteristics have been required of steel products to be used as materials for construction of these welded structures. One of the characteristics required of these steel products is the CTOD toughness, which is determined by the fracture toughness test according to BS 7448 or ASTM E 1290. Improvements in and stabilization of the CTOD toughness greatly contribute to the improvement in the safety of welded structures.
The xe2x80x9cCTOD toughnessxe2x80x9d is an indicator of the resistance to CTOD (Crack Tip Opening Displacement). More specifically, a test specimen given a fatigue crack is subjected to three point bending at a given temperature and the opening displacement at the crack tip is measured with a clip gage or the like. The CTOD toughness is evaluated in terms of the critical value of crack tip opening displacement at the time of fracture (hereinafter referred to as xe2x80x9ccritical CTOD valuexe2x80x9d).
It is known in the art that it is effective in improving the CTOD toughness of steel products to reduce the C content in the steel. To compensate for the decrease in strength as resulting from the reduction in C content in steel, various alloying elements are added and/or the production process is modified to increase the strength. Thus, for example, steel products which utilize Cu precipitation hardening are disclosed in ASTM A 710 and U.S. Pat. No. 3,692,514. These steel products are characterized by their being excellent in weldability. Improvements in their toughness in low temperature environments are still desired, however.
While the CTOD toughness is evaluated by using a plurality of test specimens collected from one steel product and carrying out a plurality of test runs under the same conditions, some test specimens may show markedly lower critical CTOD values as compared with other test specimens in certain instances even when they are tested under the same conditions. The CTOD toughness required of steel products is evaluated in terms of the lowest critical CTOD value (hereinafter referred to as xe2x80x9cminimum critical CTOD valuexe2x80x9d) among the critical CTOD values of those test specimens tested under the same conditions and, therefore, it is necessary that the minimum critical CTOD value for a steel product should clear a given value. Therefore, in the art, the C content in steel is reduced to an excessive extent or expensive alloying elements are added in large amounts to thereby excessively improve the CTOD toughness in preparation for the phenomenon mentioned above. As a result, it has been difficult to reduce the cost of production of Cu precipitation strengthened steels.
Accordingly, it is an object of the present invention to provide a Cu precipitation strengthened steel having good and stable CTOD toughness as well as a method of producing the same.
The Cu precipitation strengthened steel of the invention comprises, on the mass percent basis, C: 0.02-0.10%, Mn: 0.3-2.5%, Cu: 0.50-2.0%, Ni: 0.3-4.0% and Ti: 0.004-0.03% and further comprises Si: 0.01-0.4% and/or Al: 0.001-0.1%, with the contents of incidental impurities being P: not more than 0.025%, S: not more than 0.01%, N: not more than 0.006% and Se: not more than 0.005%, with the value of Pcm defined by the formula (1) given below being not more than 0.28:                     Pcm        ⁢                  xe2x80x83                =                  xe2x80x83                ⁢                  C          ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      Si            30                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      Mn            20                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      Cu            20                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      Ni            60                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      Cr            20                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      Mo            15                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      V            10                    ⁢                      xe2x80x83                    +                      xe2x80x83                    ⁢                      5            ⁢                          xe2x80x83                        ⁢            B                                              (        1        )            
where C, Si, Mn, Cu, Ni, Cr, Mo, V and B respectively represent the contents (in mass percent) of the respective elements.
The strength can further be improved by causing the steel to contain, on the mass percent basis, at least one element selected from the group consisting of Cr: 0.05-1.0%, Mo: 0.05-1.0%, Nb: 0.005-0.04%, V: 0.01-0.10% and B: 0.0005-0.003%.
The toughness can further be improved by causing the steel to contain, on the mass percent basis, at least one element selected from the group consisting of Ca: 0.0005-0.05%, Zr: 0.0005-0.05% and REMs (rare earth metals): 0.0005-0.05%.
The steel of the present invention can be produced by a production process comprising the following steps (a) to (e), which is given as an embodiment of the present invention:
Step (a): Heating a steel having the above chemical composition to a temperature not lower than 950xc2x0 C. but not higher than 1250xc2x0 C.;
Step (b): Hot rolling the thus-heated steel;
Step (c): Allowing the hot-rolled steel to cool or cooling the same in an accelerated manner;
Step (d): Reheating the steel after being allowed to cool or acceleratedly cooled to a temperature not lower than 450xc2x0 C. but not higher than 680xc2x0 C.; and
Step (e): Air cooling the reheated steel.
When it is intended that the stability of welded structures in which the steel product of the invention is applied as a material for constructing them be improved, the steel can also be produced by another embodiment of the production process of the present invention which comprises the following steps (A) to (C):
Step (A): Estimating, for a steel having the above chemical composition, the change in tensile strength in the process of strain-removing heat treatment on the assumption that the steel may be subjected to strain-removing heat treatment after tempering under various conditions;
Step (B): Determining the tempering conditions based on the change in tensile strength as estimated in step (A); and
Step (C): Tempering the steel under the tempering conditions established in step (B).